FIELD OF THE INVENTION
The invention relates to a method of burning fuel with air in a combustion chamber, to which the air is fed through at least one air inlet and to which the fuel is fed through a plurality of burners, each burner having an associated delay time corresponding to a period of time after which an acoustic impulse in the combustion chamber causes a thermal impulse during combustion of the fuel fed through the burner. The invention also relates to a corresponding device for burning fuel with air.
International Patent Publication No. WO93/10401 discloses such a method and such a device.
The invention relates in particular to a method and a device of the type mentioned at the outset for use in a gas turbine. A gas turbine is a combination of a compressor for air, a combustion device including at least one combustion chamber for burning a fuel in the air while forming a flue gas, and a turbine in the actual sense for the expansion of the flue gas. The turbine may be multipart, that is it may include a plurality of turbine sections connected one behind the other, and the same applies to the compressor. The compressor is a turbo compressor, in particular. Within the limits of conventional practice, the turbine drives the compressor.
The invention is associated with the task of damping or avoiding acoustic oscillations in a combustion chamber. The oscillations are induced by the combustion and are known as "combustion oscillations".
In many combustion chambers, specifically both in combustion chambers of gas turbines and in combustion chambers of boiler furnaces, industrial furnaces or other plants, unstable operating states occur under certain conditions that are clearly defined by the relevant thermodynamic operating parameters such as air coefficient and thermal output. The operating states are characterized by correlated fluctuations in the heat production during the combustion and in the static pressure in the combustion chamber and/or in the plant parts disposed upstream and downstream of the latter. Those fluctuations manifest themselves as acoustic oscillations which are self-excited in the combustion chamber. In addition to an increased noise nuisance in the vicinity of the relevant plant, the acoustic oscillations cause increased mechanical and thermal stresses on the combustion chamber and other parts of the plant. The oscillations can definitely lead to complete or partial failure within a short time.
The increased use of premix burners in corresponding combustion chambers, which accompanies increasing demands for combustion at as low a level of pollution as possible, leads to an increased tendency to form combustion oscillations due to the higher reaction density achieved by a premix burner, the ignition, which depends on the chemical composition of the mixture to be burned to a greater extent than in a diffusion burner, and the convective delay time within the flame being formed, which delay time is reduced as compared with a diffusion burner.
A combustion oscillation is generally due to an interaction between the flow of the reaction partners discharging from the burner being used and the transformation of energy during the combustion. The interaction produces and maintains a stable acoustic oscillation in combination with an acoustic resonance appearing in the combustion chamber and adjoining plant parts. In that case, there is a closed effective circuit in a system which is capable of acoustic oscillations and is formed of the burner together with feed lines, the flame itself and the combustion chamber. In that case, the energy required for forming and maintaining the acoustic oscillation is delivered from the combustion process itself.
The acoustic relationships in a combustion chamber together with attached systems are explained in detail in a book entitled "Combustion-Driven-oscillations in Industry" by A. A. Putnam, American Elsevier Publishing Company, Inc., New York 1971, page 2. Reference should also be made to a dissertation entitled "Experimentelle und theoretische Untersuchungen der Entstehungsmechanismen selbsterregter Druckschwingungen in technischen Vormish-Verbrennungs-systemen" [Experimental and Theoretical Research into the Development Mechanisms of Self-Starting Pressure Oscillations in Technical Premix-Combustion Systems] by H. Buchner, University of Karlsruhe 1992, pages 4 and 5. A condition known as "Rayleigh criterion", which has to be fulfilled so that a stable combustion oscillation can occur, is presented and explained in each case.
A criterion which relates the period of an acoustic oscillation, the possibility of occurrence of which is discussed, to a "delay time" substantially characterizing a burner and its operation can also be derived from the Rayleigh criterion. That delay time is a period of time after which an acoustic impulse in the combustion chamber, to which the burner is attached, causes a thermal impulse during the combustion of the fuel fed through the burner. The delay time is determined relative to a stable oscillation present in the combustion chamber and to a thermal oscillation produced by the stable oscillation through the burner. In other words, the delay time is a periodic fluctuation of the transformation of energy during the combustion effected by the burner, which corresponds to a phase difference between the acoustic and the thermal oscillation. In that respect, reference should be made in particular to the dissertation of Buchner, pp. 26-29 as well as to a report entitled "Untersuchung der Anregungsmechanismen selbsterregter Verbrennungsschwingungen an einem Verbrennungssytem fur Flussigkraftstoff" [Research into the Excitation Mechanisms of Self-Starting Combustion Oscillations in a Combustion System for Liquid Fuel] by J. Herrmann, P. Zangl, S. Gleis and D. Vortmeyer, VDI reports No. 1193 (1995), pp. 251-260.
The delay time of a burner in a combustion chamber is composed of various summands, which in each case can be attributed to individual components of the system being formed of the burner, the combustion chamber and the flame. The summands which can be related to the burner and the combustion chamber are determined mainly by the geometry of the burner and the combustion chamber. A summand which can be attributed to the flame itself is substantially determined by the properties of the combustion itself. The summand itself can be broken down further into a "convective delay time", which characterizes a transport time for the transport of the reaction partners to the flame front, where the combustion starts, a "heating time", which specifies the time for heating the reaction partners to the temperature required for ignition, and a "reaction-kinetic delay time", which is determined by the course of the combustion itself. As a rule, the convective delay time clearly outweighs the other two summands.
Conventional measures for suppressing a combustion oscillation in a combustion chamber are based either on the fact that more or less empirically passive acoustic measures are used, such as choke points, resonators and/or silencers, as is seen in the above-mentioned book by Putnam, pp. 156-175, or on the fact that the feeding of the fuel is carried out with active modulation with the aim of uncoupling the energy release from acoustic oscillations in the combustion chamber. Such a measure is designated as "active instability control". For an explanation, see a paper entitled "Die `aktive Instabilitatskontrolle`, als Untersuchungsmethode fur selbsterregte Verbrennungsinstabilitaten" [The "Active Instability Control", as Research Methods for Self-Starting Combustion Instabilities] by S. Gleis and D. Vortmeyer, in VDI Reports No. 765 (1989), pp. 645-656. In German Published, Non-Prosecuted Patent Application DE 42 41 729 A1, an actuator is described, through the use of which a mass-flow or pressure fluctuation can be imposed on a liquid flow under pressure. The actuator is proposed for use in the active control of combustion instability in liquid fuel burners and in devices for the atomization of liquids.
The conventional passive measures for suppressing combustion oscillations are used to stabilize the operation of the plant by displacing the acoustic properties of subsystems in such a way that combustion oscillations no longer occur over the entire desired operating range. Those measures require devices which have to be adapted to the respective plant in the individual case and always involve the risk that, although known unstable operating points will be stabilized, further instability will be caused under different operating conditions.
A device for reducing oscillations in combustion chambers is specified in German Published, Non-Prosecuted Patent Application DE 43 36 096 A1. In that device, a plurality of burners are disposed in the direction of flow upstream of the combustion chamber. In each case, adjacent burners are disposed in such a way as to be displaced relative to one another by a predetermined distance in the direction of flow.
In that case, the predetermined distance is selected in such a way that, during operation of the burners, the temperature fluctuations of adjacent burners spreading in the direction of flow are exactly opposite. In a cross-section relative to the direction of flow, combustion zones having a positive and a negative deviation from a mean temperature therefore lie next to one another. In that case mixing of those regions takes place in the direction of flow and thus the temperature is evened out. That is intended to prevent a combustion oscillation induced by temperature fluctuations and thus also, due to different densities, pressure fluctuations.
Active measures for suppressing combustion oscillations can only be realized in industrial plants at a high cost, in particular when a gaseous fuel is to be used, and in addition are susceptible to trouble and need maintenance. Furthermore, they merely lead to damping of the instability present in each case and are greatly restricted in their effectiveness by the decisive structural conditions of the plant in the respective individual case.